If there is one thing humanity will ever wish for is to avoid old age and stay young forever. Fountains of youth and mythology of longevity are common in our legends and fairy tales. Aging is something that we may not be able to avoid as human, it comes whether we like it or not -- but still, we would like to avoid it if we have the chance
As a sort of bitter joke, we can look to some of the other creatures on Earth and find that they have what we wish for. Some refer to these animals, perhaps mistakenly, as biologically immortal. But they can die anyway but not due to aging. They keep their youthful vigor right up until some other form of death takes them.
Marine Sponges
As far as multicellular animals go, marine sponges are as simple as they come. They don’t possess the common elements that other animals (like us) have in abundance. They have no internal organs, digestive tracts, or nervous and muscular systems.
Yet a marine sponge specimen has been found to be at least 11,000 years old, with some studies suggesting the potential life span of these creatures to be in the hundreds of thousands of years. The key to their longevity may lie in their simplicity.
Andrey Lavrov and Igor Kosevich, biologists who studied marine sponges’ abilities, found that when the sponges were subjected to tissue dissociation (by mechanically or chemically separating the cells from one another), the sponges were able to re-form into their original shape.
The biologists reported, “In a number of cases, such multicellular aggregates may result in a full reconstruction of an animal’s initial organization.†These amazing regenerative abilities render these creatures nearly ageless.
Planaria (Flatworm)
Our school classes taught us about the special regenerative powers of the flatworms, unassuming creatures that harbor the ability to regrow into two healthy flatworms if cut in half. Recently, a lab at MIT performed an experiment that regrew an entire flatworm from a single cell. But one cause of this remarkable feat may not have been mentioned in our high school classes.
Dr. Aziz Aboobaker of The University of Nottingham commented:
Usually when stem cells divide—to heal wounds or during reproduction or for growth—they start to show signs of aging. This means that the stem cells are no longer able to divide and so become less able to replace exhausted specialized cells in the tissues of our bodies. [ . . . ] Planarian worms and their stem cells are somehow able to avoid the aging process and to keep their cells dividing.
Due to this cellular youth, flatworms defy aging, making it difficult to accurately measure a flatworm’s life span.
Hydra
This creature is only about 2.5 centimeters (1 in) high and treelike. To understand its place on our list, we need to understand the concept of senescence, which is the increase in mortality rate as a creature gets older.
For example, humans are more likely to die the older we get. However, the Hydra does not have this increase in mortality. (Except for one species, Hydra oligactis. Sorry, buddy.)
They accomplish this feat using three distinct types of stem cells, which are initially undifferentiated and can eventually become many different specialized cell types. These stem cells are actively renewing the body of the Hydra and thus fighting off any aging process that could lower their mortality rate.
In a laboratory setting, it’s estimated that 5 percent of Hydra could live up to 1,400 years using this process.
Lobster
Often caught to be eaten as delicacies, these bottom-feeding ocean animals have indeterminate growth, which means they have no maximum size. As a result, the longer a lobster lives, the bigger it will get—with no biological process to halt that growth.
The heaviest lobster ever caught was a little over 20 kilograms (44 lb). It was found off the coast of Nova Scotia. Estimates of these animals’ life span range from 50 to 100 years—not much different from that of humans. But the fascinating thing about lobsters is how they age and die. They show no loss of appetite, sex drive, energy, or metabolism as they get older.
However, lobsters do make it hard to measure their age. They grow in a process of molting, where they shed their entire exoskeleton. After each molting, all hard surfaces of the animal are discarded. So there is nothing left that can be aged with accuracy.
It is that process of molting that eventually kills them and not the usual aging process that humans face. The larger a lobster becomes, the more dangerous molting is and the more energy it takes.
Eventually, a large lobster will no longer be able to survive the process or even muster up enough energy to begin it, thanks to its giant size. If this hurdle were somehow removed, there’s no telling how long these creatures could live.
Midland Painted Turtle
Although many turtles show a long life span and lack of senescence, Blanding’s turtles and midland painted turtles have a seemingly backward take on aging. The elderly females lay even more eggs than their younger counterparts and die at a lower rate. They become more likely to keep living with age.
The longevity of the entire turtle family is impressive. One giant tortoise lived to at least 250 in the Calcutta zoo. Dr. Christopher J. Raxworthy from the American Museum of Natural History said plainly, “Turtles don’t really die of old age.†Instead, the internal organs of elderly turtles are almost identical to those of their teenage counterparts.
This impressive life span is also shown in how slowly female turtles reach sexual maturity—only after 40 or 50 years in some species. Dr. Raxworthy added that if it weren’t for getting crushed by an automobile or falling prey to a disease, a turtle could live indefinitely.
Turritopsis Dohrnii (Jellyfish)
Imagine that we could decide on a whim to reverse our aging at any point in our lives. Our hair would get less gray, our wrinkles would disappear, our bodies would get stronger, and our metabolism would increase. We’d be like teenagers again and then like children and, finally, like babies.
Then we’d start aging again from the beginning, living our lives a second time none the worse for wear. This is the exact situation of the Turritopsis dohrnii, the immortal jellyfish. At any point in its life, the jellyfish can reverse aging in a process known as transdifferentiation, revert to its earliest form, and then continue living from there. So far as we know, this process can continue without end.
This mechanism can be triggered by anything from mutilation to old age, starvation, or disease. If left to their own devices, these jellyfish will rejuvenate about 10 times over the course of two years.
Sometimes, these events happen less than a month apart. Also a proficient hitchhiker, this everlasting jellyfish has spread to every ocean in the world by hitching rides on cargo boats.